The present invention relates to a process for the preparation of 1-hexene by the oligomerization of ethylene in the presence of vanadium complexes with a low oxidation state.
The possibility of preparing olefins with a double terminal bond by means of the oligomerization of lower olefins, for example 1-hexene by the trimerization of ethylene, is already known: for example U.S. Pat. No. 4,668,838 describes a process involving the use of a catalyst which comprises the reaction product between a chromium compound, a hydrocarbon derivative of partially hydrolyzed aluminum and a ligand-donor compound; European patent application Ser. No. 416,304 involves, in the process in question, the use of a catalytic system comprising a pyrrolidic compound of chromium and a compound of aluminum; European patent application 537,609 relates to the process of interest which is carried out in the presence of a catalyst consisting of a complex of chromium with a polydentate ligand and an aluminoxane.
In addition to the above references, others can be cited relating to the use of catalytic systems based on compounds of chromium; preparation processes of 1-hexene by the oligomerization of ethylene in the presence of these catalytic systems, however, seem to have various drawbacks such as, for example, insufficient activity of the catalyst or difficulty in its preparation or the low selectivity with respect to the desired product.
The Applicant has now found that it is possible to effect the synthesis of 1-hexene by the oligomerization of ethylene using a process which overcomes all the disadvantages which negatively characterize analogous processes described in the state of the art above, and which consequently seem to have all the necessary industrial requisites. The object of the invention, according to the present patent application, therefore relates to a process for the preparation of 1-hexene which consists in subjecting ethylene to oligomerization in the presence of a catalyst comprising a vanadium complex with a low oxidation state selected from those included in the follow formula:
(arene)2VX
wherein the term arene represents benzene or mono-, di-, or tri-alkylsubstituted benzene, V is a vanadium ion with a low oxidation state, and X is an anion selected from non-coordinating anions such as B(Ar)4xe2x88x92, AlCl4xe2x88x92, carboxylates and sulfonates.
The following complexes have proven to be particularly advantageous:
bis-(mesitylene)-vanadium trifluoroacetate
bis-(mesitylene)-vanadium-tetraphenylborate
bis-(mesitylene)-vanadium-tetrachloroaluminate.
Vanadium arenes are already known in the art and can be prepared for example according to the indications, which form an integral part of the present invention, contained in the articles of E. O. Fischer and U. S. Kogler, Chem. Ber. 90, 250 (1957), and F. Calderazzo, Inorg. Chem., 3, 810 (1964): for example according to the schemes of Table 1.
In accordance with a wider aspect of the process of the present invention, these vanadium complexes can be used for the oligomerization of olefins other than ethylene, such as, for example, propylene, 1-butene, etc.: the reason the Applicant has decided to limit the description of the invention to the preparation of 1-hexene alone is simply because of the specific industrial interest in the production of this compound: any expert in the field will be able to understand from this description how to effect the oligomerization reaction starting from olefins other than ethylene, this process being included, however, in the scope of the invention according to the present patent application.
The oligomerization reaction of ethylene, according to the process of the present invention is carried out in the absence of co-catalysts, in the presence of the above vanadium compound alone or in the presence of the vanadium compound and a heterocyclic compound selected from pyrrole and derivatives.
When this heterocyclic compound is adopted, it is used in a ratio, with respect to the vanadium complex, ranging from 1 to 20: heterocyclic compounds which can be advantageously used are pyrroles, pyrazoles, phenanthroline, pyridine and other mono and polynitrogenated derivatives.
The oligomerization reaction of ethylene, according to the present invention, is carried out in the presence of a solvent which is normally selected from aromatic hydrocarbons and cycloaliphatic hydrocarbons: of these, the use of toluene and cyclohexane has proved to be particularly advantageous.
The reaction is carried out at a temperature ranging from 0 to 150xc2x0 C., preferably from 25 to 60xc2x0 C., and at a pressure ranging from 1 to 100 atm., preferably from 1 to 50 atmospheres. The reaction time is normally less than 36 hours, preferably from 1 to 24 hours. At the end of the reaction, 1-hexene can be separated and recovered from the reaction mixture using techniques well known to experts in the field.